Flexible tubular pipe

ABSTRACT

It is of the type comprising at least one metal profile (P) spiralled in a helix about a longitudinal axis (A-A) of the said flexible pipe and is characterized in that the profile (P) consists of a metal strip ( 1 ), each turn of which comprises two separate box sections ( 2, 3 ) which are obtained by bending the said strip so that the free edges ( 8, 9 ) of the strip are turned down between the two box sections ( 2, 3 ) and are in contact with a strip portion ( 6 ) provided on a bulge ( 5 ) formed between the two box sections, the said contact portion ( 6 ) lying approximately along the neutral fibre ( 7 ) of the said profile, and in that each box section ( 2, 3 ) has a trapezoidal shape.

[0001] The present invention relates to a flexible tubular pipe whichcan be preferably used in deep-sea applications, for depths of between1000 and 3000 m, although it can also be used for depths of less than1000 m. Such flexible tubular pipes are used in subsea oil productioninstallations for transporting fluids such as hydrocarbons.

[0002] Several types of flexible tubular pipes are used at the presenttime and are described in API (American Petroleum Institute) 17 J. Incertain flexible pipes, there is a pressure vault which consists of ahelical winding with a short pitch of a shape to wire which may beself-interlockable or interlockable by means of a fastener. Likewise,the metal carcasses used in flexible pipes called “rough bores” areformed from a crush-resistant doubly interlocking profiled metal strip.

[0003] In all cases, it has been attempted to improve the moment ofinertia/weight ratio of the interlocked strips or shaped wires used forproducing the various metal layers of the flexible pipes.

[0004] For deep-sea applications and in the case of pressure vaults, thereinforcing wires must have a high moment of inertia in order towithstand the external pressure and a low weight in order to reduce thetotal weight of the flexible pipe so as to improve the performance ofthe pipelaying means and allow the flexible pipe to be self-supporting.

[0005] Several solutions have been proposed.

[0006] A first solution has consisted in using a shaped wire, thecross-section of which is in the form of an I, as described in FR-A-2782 142. Such a shaped wire has an acceptable moment of inertia/weightratio but the manufacturing cost is very high because of the fact thatit is obtained by rolling or wire drawing.

[0007] Another solution is described in FR 2 654 795. The internalcarcass is formed from doubly interlocking metal strips, by making aflat metal tape, such as a stainless steel strip, undergo plasticdeformation in order to give it the shape of a doubly interlockingprofiled strip, and then by spiralling the profiled strip, that is tosay winding it helically with a short pitch with interlocking of theturns of the profiled metal strip. After two consecutive turns have beeninterlocked, a final plastic deformation of the strip is carried out inorder to complete the interlocking.

[0008] In Patent FR 2 665 237 it is recommended to produce a tubularmetal carcass comprising at least one box section wound in a helix witha short pitch, the said metal carcass being obtained by means of twocomplementary profiled strips wound helically with a short pitch. Manyexamples of profiled strips are described and shown in that document,some of which, such as for example those in FIGS. 8 and 9, consisting ofa strip in the form of an elongated S and having a box section at afirst end and an upwardly curved fastening edge at the other end, thefastening edge penetrating a dish formed by the box section and thetransverse bar of the S of the preceding turn. The curved fastening edgemay rest on the bottom of the dish (FIG. 9) or it may not be in contactwith the said bottom (FIG. 8).

[0009] It should be noted that all the cross-sections of the boxsections provided at one or both ends of each profiled strip are squareor rectangular cross-sections.

[0010] Although such box-section profiled strips have been satisfactory,they have been found to have certain drawbacks. When one considers thata box section is formed by parts of the same turn of the profiled stripand comprises an upper wall, a lower wall and side walls and when anexternal force is applied to one of the upper and/or lower walls, suchas a compressive or crushing force or else a force generated by thepressurized fluid, buckling of the side walls or faces and/or the upperor lower walls of the box section may then occur, thereby reducing, atleast locally, the crush resistance of the internal carcass.

[0011] For forces or pressures exceeding a certain value, buckling ofthe side walls of the box section occurs.

[0012] For lower forces or pressures and when the side walls are notstrictly perpendicular to the upper and lower walls of the box section,crushing of the said box section may occur, resulting in the side wallsmoving further apart or closer together (opening or closing of the boxsection). To prevent this opening or closing phenomenon, it has beenrecommended in certain cases such as, for example, in FR 2 665 237 toweld one end of the box section at one point on the strip. However, sucha solution increases the manufacturing cost and it is difficult toimplement.

[0013] In a prior embodiment, the box-section strip adopts the shape ofa pair of spectacles. This profile is produced from a strip whose freeedges are welded to this same profile so as to give the profile goodstability for laying. However, the welding operation is difficult toimplement and considerably increases the manufacturing cost. Because thewelds are off-centre with respect to the neutral fibre of the profile,they are subjected to high stresses during the spiralling. This mayresult in a local fracture of the weld. Such a fracture of the weld may,in turn, result in a considerable reduction in the mechanicalperformance of the pressure vault, of around 10 to 30%, because of thefact that the profile is no longer stable.

[0014] In another prior embodiment, the spectacles-shaped profile issimilar but produced from two tubes, of square cross-section, the saidtubes being joined together by a strip which is welded to these tubes.The welding operations again considerably increase the manufacturingcost.

[0015] In addition, these various shapes of the profile do not make itpossible to limit the creep of an adjacent sealing sheath. It is soughtto prevent such creep as far as possible, mainly in the case of thepressure vault. It should also be noted that no information is providedregarding the width/thickness ratio of the strip, although this hasbecome important in order to ensure stability of the profile during pipelaying, so as to prevent the buckling effect.

[0016] This is because if the profile has too large a width with respectto its thickness, the side walls of the box section will buckle duringspiralling or winding with a short pitch.

[0017] In patent application Ser. No. 00/05,175 of 21 Apr. 2000, theprofiles that are described give good results but sometimes havedrawbacks, especially because of the great dissymmetry of the profile,this dissymmetry constituting an obstacle during spiralling, which isconsequently difficult to carry out. In addition, the moment ofinertia/pitch ratio is reduced by the presence of the free edge whichprovides little moment of inertia.

[0018] The object of the present invention is to provide a profile whichmakes it possible to simplify the spiralling operations while stillhaving a high moment of inertia/pitch ratio, similar to that obtainedwith conventional shaped wires of equivalent height, and a high momentof inertia/weight ratio which is needed in particular for great depths.

[0019] The subject of the present invention is a flexible tubular pipe,of the type comprising at least one metal profile spiralled in a helixabout a longitudinal axis (A-A) of the said flexible pipe and ischaracterized in that the profile consists of a metal strip, each turnof which comprises two separate box sections which are obtained bybending the said strip so that the free edges of the strip are turneddown between the two box sections and are in contact with a stripportion provided on a bulge formed between the two box sections, thesaid contact portion lying approximately along the neutral fibre of thesaid profile, and in that each box section has a trapezoidal shape.

[0020] One advantage of the present invention lies in the fact that useis made of a metal strip which is not very costly and lends itself wellto being bent in order to form separate box sections and thus to realizea profile which has a high moment of inertia, while still maintaining ahigh moment of inertia/weight ratio when the profile is used inparticular for producing a pressure vault for large-depth applications.The bending of the free edges of the strip so as to come into contactwith a strip portion lying approximately along the neutral fibre of theprofile considerably increases the stability of the profile and makes iteasier for it to be spiralled.

[0021] Further features and advantages will become more clearly apparenton reading the description of several embodiments of the invention andthe appended drawings in which:

[0022]FIG. 1 is a cross-sectional view of part of the profile accordingto the invention, the profile being interlocked from above;

[0023]FIG. 2 is a cross-sectional view of part of the profile accordingto the invention, the profile being interlocked from above and below;

[0024]FIG. 3 is a cross-sectional view of part of the profile accordingto another embodiment;

[0025]FIG. 4 is a cross-sectional view of the profile interlocked withan improved fastener.

[0026] The profile P according to the invention shown in FIGS. 1 and 2consists of a metal strip 1 which is bent so as to form two separatetrapezoidal box sections 2, 3 which are symmetrical with respect to avertical axis of symmetry B-B. The box sections 2, 3 are separated by anupper bulge 4 and a lower bulge 5. The strip portion 6 which constitutesthe bottom of the lower bulge 5 lies approximately in the plane of theneutral fibre 7 and the free edges 8, 9 of the strip which have beenturned down inwards during the bending bear on the said strip portion 6,the free edges 8, 9 being aligned and placed opposite each other, asshown in the figures.

[0027] Each trapezoidal box section 2, 3 has side walls 10, 11 which areinclined and make an angle cc greater than 60° and less than 90° withthe horizontal C so that, owing to the effect of the contact pressuresto which the profile is subjected, the free edges 8, 9 will have atendency to move closer together, sliding over the contact strip portion6, without any possibility of them overlapping each other, somethingwhich could result in deformation of the profile and therefore indestabilization. This is because, owing to the effect of the contactpressures developed in the flexible pipe and transmitted to the profile,the free edges 8, 9 can butt against each other, thereby preventing themfrom overlapping each other and from further stabilizing the profile.Since the free edges 8, 9 are on the contact portion 6, which liesapproximately in the plane of the neutral fibre 7 and therefore in anapproximately central region of the profile, the said region willtherefore be subjected to few stresses during spiralling and will notwork very much.

[0028] This greatly improves the stability and the performance of theprofile. For an angle α close to 90°, the profile is stronger and for anangle α close to 60° the profile is more stable. Consequently, theoptimum angle α is a compromise between strength and stability.

[0029] The profile described above can be used for producing a pressurevault or the metal carcass, when the flexible pipe has one, by windingit in a helix, with a short pitch, about the horizontal axis A-A of theflexible pipe, each turn of the winding consisting of a pair of boxsections 2, 3. Since the profile is not self-interlockable, it ispossible to interlock the turns in several ways.

[0030] The first way of interlocking is shown in FIG. 1. In thisembodiment, the turns of the winding of the profile are interlocked fromabove, that is to say the fasteners 12, for example in the form of anupside-down U, have their arms 13 and 14 placed in the consecutive upperbulges 4. Another method of interlocking, not shown, consists in usingthe same fasteners, but referenced 12′, which have their arms 13′ and14′ placed in the consecutive lower bulges 5. In the latter case,interlocking is obtained from below, as opposed to the previous waywhich is called interlocking from above.

[0031] Another method of interlocking, shown in FIG. 2, consists ininterlocking the turns from above and from below, with upper fasteners12 whose arms 13 and 14 are placed in the upper bulges 4 and lowerfasteners 12′ whose arms 13′ and 14′ are placed in the lower bulges 5.

[0032] Interlocking from below has the advantage of not inducing anystresses on the free edges 8 and 9.

[0033] The upper and/or lower fasteners may advantageously have, in theinterjoint space 15 between two consecutive turns, a bulge or hump 16which makes it possible to increase the local moment of inertia of thefastener 17. The fastener, stiffened at the hump 16, is more resistantto the internal pressure of the fluid flowing in the flexible pipe.Creep of the inner sealing sheath, owing to the effect of the internalpressure, applies a high contact pressure on the fastener. Moreover, toimprove the technical characteristics of the fastener 17, the radii ofcurvature of the hump 16 correspond to those of the profile so as toallow the functional clearances of the vault to be maintained. Toincrease the burst strength of the pressure vault owing to the effect ofthe internal pressure of the fluid flowing in the flexible pipe, it ispossible to use the properties of the fastener 17. Since the resistanceto the internal pressure depends in part on the cross-section and on themechanical properties of the material used, all that is required is toincrease the thickness of the fastener or to choose a material havinghigh mechanical properties, preferably properties superior to those ofthe profile; the vault-fastener pair will have a higher burst strength.

[0034] It is also possible to use means for reducing the creep of animpermeable inner polymeric sheath which bears on the profile. Thesemeans may comprise, by themselves or in combination, a rod 18, which ispreferably placed in the lower bulges 5, and/or a shaped wire 19, whichcovers the interjoint space 15 between two consecutive turns. The shapedwire may be flat or have the shape of an inverted T 20, as shown in FIG.3, the vertical arm 21 of the T 20 fitting into the interjoint space 15.The shaped wire 19 may also be an anti-creep woven strip like thatdescribed in FR 2 744 511. These means for reducing the creep of asheath may be provided above the profile when it is interlocked frombelow and when the polymeric sheath is placed above the profile.

[0035] Another advantage lies in the fact that since the interlocking iscarried out in the box section, the moment of inertia/pitch ratio of theprofile is thus optimized while eliminating the regions of low moment ofinertia.

[0036] The annulus of a flexible pipe is bounded by the outer sealingsheath and the inner sealing sheath. This annulus is generally atatmospheric pressure. When this annulus is invaded by sea water, as aresult of a tear or perforation in the outer sealing sheath or when thelatter does not exist, the hydrostatic pressure is applied directly tothe inner sealing sheath. As a result, the contact pressures to whichall the armour plies lying above the inner sealing sheath are subjectedare reduced and, in this case, they will no longer have to withstand alarge pressure differential and consequently no longer take up thisforce, which is transferred to the internal carcass. The armour plieswill only have to take up the internal pressure of the fluid flowing inthe flexible pipe, reduced by the hydrostatic pressure, together withthe axial forces and the compression due to the tension.

[0037] The free edges 8, 9 are slightly apart from or in contact witheach other, so as to limit the deformation of the profile when it issubjected to contact forces, or they may be joined together by alongitudinal weld, by a spot weld, by bolting or by crimping with aT-shaped fastener.

[0038] When the annulus is invaded by sea water, if the profile isimpermeable (longitudinal seam weld), the hydrostatic pressure isapplied to the outer surface of the profile and requires the latter tobe designed so that it withstands a pressure differential. Since the twobox sections of the profile each have a small volume, the crushresistance is increased compared with a single box section having alarger volume. When the profile is not impermeable, in the case of animmersed annulus, the water also invades the inside of the profile andthe box sections of the profile are therefore no longer subjected to thehydrostatic pressure.

[0039] In addition, the upper and lower bulges reduce the risk of theupper and lower walls of the profile box sections bending.

[0040] The profiles shown in the figures must not exceed a certain widthL so that the hollow profile can be wound helically. This is because ifthe profile is too wide, during winding the forces will be too high andthere would be a risk of making the side walls of the profile buckle.The profile must therefore have a maximum width L which depends, on theone hand, on the thickness e of the strip and, on the other hand, on theheight H of the profile. Tests carried out have shown that the resultsare satisfactory when:

0.5<L/H<4;

L/e<20.

[0041] By producing a reinforcing wire from a strip according to theinvention, a more compact profile-fastener pair is obtained, therebyoptimizing the moment of inertia/winding pitch ratio. As a consequence,the performance of the pressure vault produced in one of the embodimentsdescribed above is increased. The measurements carried out show that,for a profile 20 mm in height and 3 mm in thickness, the moment ofinertia/effective pitch ratio is 260 mm³ whereas the same ratio for abox-section profile of the prior art, 20 mm in height and 3 mm inthickness, is 210 mm³.

[0042] Production starts with a strip 2.5 mm in thickness and 120 mm inwidth. The strip is bent so as to obtain a profile 20.4 mm in height and38 mm in width. The slopes of the side faces of the profile are 80° tothe horizontal. With such dimensions, a moment of inertia/pitch ratio of250 mm³ is obtained.

[0043] As regards the moment of inertia/weight ratio of the profileaccording to the invention, it was found that this was 20% highercompared with known lightened profiles and 60% higher compared withshaped wires (zeta, teta). The present invention thus achieves twoobjectives, namely a reduced manufacturing cost and an increase in themoment of inertia/weight ratio for deep-sea use in which it is necessaryto withstand high external pressures.

[0044] It should be noted that the rod 18 described above may consist ofa plastic rod or a tube conveying a heat-transfer fluid, for activeheating for example, or containing a power cable for active heating. Therod or tube placed in the bulges may also contain an optical fibre.

[0045] The conveying functions mentioned above, and relating inparticular to active heating, the optical fibre, etc., may beincorporated directly into at least one of the box sections of theprofile.

1. Flexible tubular pipe, of the type comprising at least one metalprofile (P) spiralled in a helix about a longitudinal axis (A-A) of thesaid flexible pipe, characterized in that the profile (P) consists of ametal strip (1), each turn of which comprises two separate box sections(2, 3) which are obtained by bending the said strip so that the freeedges (8, 9) of the strip are turned down between the two box sections(2, 3) and are in contact with a strip portion (6) provided on a bulge(5) formed between the two box sections, the said contact portion (6)lying approximately along the neutral fibre (7) of the said profile, andin that each box section (2, 3) has a trapezoidal shape.
 2. Flexiblepipe according to claim 1, characterized in that the box sections (2, 3)are symmetrical with respect to a mid-axis (B-B) passing between theopposed free edges (8, 9).
 3. Flexible pipe according to either ofclaims 1 and 2, characterized in that the side walls (10, 11) of eachbox section make an angle (α) greater than 60° and less than 90° withthe horizontal (6).
 4. Flexible pipe according to claim 1, characterizedin that the free edges (8, 9) are in contact with each other. 5.Flexible pipe according to claim 1, characterized in that twoconsecutive turns of the profile wound in a helix are interlocked by afastener (12, 12′, 17), the ends (13, 14, 13′, 14′) of which fit intothe bulge (4) located above the free edges (8, 9).
 6. Flexible pipeaccording to claim 1, characterized in that two consecutive turns of theprofile wound in a helix are interlocked by a fastener (12′), the endsof which fit into the bulge (5).
 7. Flexible pipe according to claim 1,characterized in that two consecutive turns of the profile wound in ahelix are interlocked by a first fastener (12), the ends (13, 14) ofwhich fit into the bulge (4) located above the free edges (8, 9), and bya second fastener (12′), the ends (13′, 14′) of which fit into the bulge(5).
 8. Flexible pipe according to any one of claims 1 to 7,characterized in that the free edges (8, 9) are joined together. 9.Flexible pipe according to one of the preceding claims, characterized inthat a rod (18) is placed in the bulges (5).
 10. Flexible pipe accordingto any one of claims 1 to 8, characterized in that an anti-creep means(19) is placed in an interjoint space (15) provided between twoconsecutive turns of the profile.
 11. Flexible pipe according to one ofclaims 5, 7 or 10, characterized in that the lower fastener (12′, 17)has a hump (16) which is placed in the interjoint space (15) between twoconsecutive turns of the profile.